Some proposals for the removal of oil from the surface of the sea involve confinement of the oil by a floating barrier (otherwise called a boom) followed by separation of the oil confined by the barrier from the water on which the oil is floating. Such separation can be effected by, for example, skimming devices or by use of oleophilic hydrophobic materials. Floating barriers can become difficult to manipulate in rough seas and, as wave height increases, they lose their effectiveness in confining oil. Usually, these proposals involve the use of more than one self-propelled craft. Difficulties can be experienced in co-ordinating the activities of the craft. The expenses of recovering oil tends to increase with the numbers of craft used. Proposals for the removal of oil unconfined by barriers tend to involve devices which extend considerable distances over the water surfaces in order to provide a satisfactory rate of oil collection. One proposal is described in U.S. Pat. No. 3,847,816 and takes the form of a "harvesting ring" and skimming device. An alternative method described in U.S. Pat. No. 3700593, involves deploying on the surface of the water a continuous loop of material which adsorbs oil in preference to water onto its surface and advancing the loop through a desorbing means. Skimmers operate far less effectively in heavy waves than in calm water. Continuous loops and harvesting rings which are big enough to deal with large oil slicks in the open sea are difficult to control unless the sea is comparatively calm. Often, more than one self-propelled craft is required.